(Lisp people, mainly:)
computes the value of BODYFORM, executes the UNWINDFORMS, and returns the value of BODYFORM. If the computation of BODYFORM exits "nonlocally" (e.g. by throwing) then there is no return value of BODYFORM, but UNWINDFORMS get executed anyway.

unwind-protect is useful for controlling resources. For instance, it's the place to ensure open files are closed after use. A prog1 form (Scheme's begin) does not work for this when a nonlocal exit occurs. In this respect, unwind-protect is similar to the practice of freeing resources in C++ object destructors -- the destructor gets called during throw() too. So unwind-protect is the procedural counterpart of "resource acquisition is initialization".

If the same invocation of BODYFORM can enter exit multiple times, it's not clear how UNWINDFORMS should be executed, or even how many times! So languages with call/cc (like Scheme) usually don't implement unwind-protect, or do not implement it in a particularly consistent and orthogonal fashion.

unwind the stack = U = up

unwind-protect n.

[MIT: from the name of a LISP operator] A task you must remember to perform before you leave a place or finish a project. "I have an unwind-protect to call my advisor."

--The Jargon File version 4.3.1, ed. ESR, autonoded by rescdsk.

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